Cooled temperature sensitive oscillator

ABSTRACT

A temperature sensor of the fluidic oscillator type wherein the oscillator housing is cooled to permit the measurement of temperature exceeding the melting temperature of the housing.

United States Patent H 1 [111 3,707,979

Zoerb 1 Jan. 2, 1973 s41 COOLED TEMPERATURE SENSITIVE 3,451,269 6/1969Johnson ..73/3s7 OSCILLATOR OTHER PUBLICATIONS Edward G. Zoerb,Roseville, Minn. Assignee: Honeywell Inc., Minneapolis, Minn. Filed:Feb. 28, 1969 Appl. No.: 803,482

Inventor:

US. Cl ..137/81.5, 73/339 Int. Cl. ..Fl5c 1/08 Field of Search..137/81.5; 73/357, 361

References Cited UNITED STATES PATENTS Gerrard et a1. ..73/361 A NewMethod for Determining the Static Temperature of High-Velocity GasStreams, by J. A. Clark and W. M. Rohsenow, Transaction of the ASME,Vol. 74, Feb. 1952, pages 221-225, 73-357.

Primary Examiner-Samuel Feinberg Attamey--Charles J. Ungemach, Ronald T.Rciling and Charles L. Rubow [57] ABSTRACT A temperature sensor of thefluidic oscillator type wherein the oscillator housing is cooled topermit the measurement of temperature exceeding the melting temperatureof the housing.

7 Claims, 3 Drawing Figures PAIENTEDJAu 2197s 3. 707. 979

sum 1 or 2 INVENTOR. EDWARD G. ZOERB BY/PJT Z A.

ATTORN COOLED TEMPERATURE SENSITIVE OSCILLATOR BACKGROUND OF THEINVENTION This invention relates generally to fluid handling apparatusand more specifically to pure fluid temperature responsive oscillators.

It has become desirable, and frequently necessary, in an increasingnumber of applications to measure the temperature of very hot fluids.Such applications include measuring the temperatures of combustion gasesin internal combustion engines, measuring the temperatures of gases athypersonic flow velocities and measuring plasma temperatures. In many ofthese cases, it is necessary to measure temperatures exceeding themelting temperature of materials suitable for constructing temperaturesensors. In many other cases, although the temperature being measureddoes not exceed the melting temperature of the sensor body, thetemperatures are so high that materials suitable for constructingtemperature sensors oxidize and erode very rapidly. In addition to thehigh temperature requirements of these applications, it is frequentlyfurther necessary that the temperature sensor have very fast responseand/or that it be sufficiently rugged to operate reliably in otherwisevery severe environments.

As examples of materials problems encountered in attempting to constructsuch sensors, it has been found that ceramic materials may be used onlyin applications invovling slow temperature changes where thermal shockis not a problem. Conversely, although refractory metals are capable ofwithstanding'thermal shock, they can be used only in non-oxidizingatmospheres.

Various other devices have been developed as a result of requirementsfor measuring very high temperatures.

These devices include optical pyrometers, cooled thermocouples and verymassive thermocouples. Although these and certain other prior arttemperature measuring instruments are capable of measuring very hightemperatures, each of them suffers from certain disadvantages that limitits usefulness.

For example, optical pyrometers may be used to measure high temperatureswith very good accuracy. However, an optical pyrometer is bulky andcomplex, it must be located remotely from the source of the temperaturebeing measured, and it has extremely poor time response.

Cooled thermocouples are not, in themselves, bulky or complex and can belocated directly in a hot fluid whose temperature is being measured, butthey are difficult to calibrate accurately. This difficulty stems fromthe fact that the signal produced by the thermocouple is directlydependent upon the thermocouple junction temperature. In a cooledthermocouple the junction temperature is determined both by thetemperature being measured and the temperature of the cooling medium.Establishment of an accurate calibration depends upon constantradiation, conduction and convection rates. As a thermocouple approachesa high temperature, it discolors, thus changing its. radiation rate, andhence its calibration. Further, if the velocity of the fluid whosetemperature is being measured changes, the conduction and convectionrates change because of changes in the stagnation velocity andstagnation temperature. Hence, additional errors are introduced into themeasurement, necessitating the use of complex correction techniques.

Massive thermocouples relay on ablation of the thermocouple material toprevent excessively rapid destruction during the measurement of extremetemperatures. Such a thermocouple is simple, rugged and inexpensive, buthas an extremely slow time response because of the thermal inertia dueto its mass. In addition, such a thermocouple must be replacedfrequently because of the continual oxidation and erosion of thematerial from which it is constructed.

Because of the disadvantages of priorart high temperature measuringinstruments, a present need exists for an accurate, simple and ruggedhigh temperature measuring instrument having fast time response.Experience with prior art high temperature sensors (particularly, thecooled thermocouple) has led to the conclusion that a cooled temperaturesensor is unsuitable for making temperature measurements where accuracyand fast time response are required. The applicant has, however,discovered certain characteristics of specially constructed temperaturesensitive fluidic oscillators which permit them to be cooled withoutintroducing prohibitive errors into the temperature measurement.Further, the applicant has provided such a temperature sensitive fluidicoscillator which incorporates special cooling features so as to providean accurate, rapidly responding temperature sensor capable of measuringvery high temperatures.

SUMMARY OF THE INVENTION Briefly, the applicant has discovered that theresponse of certain specially constructed temperature sensitive fluidicoscillators is only weakly affected by the temperature of the oscillatorbody. The applicant s invention comprises such a fluidic oscillatorformed in a housing and means for maintaining the housing at atemperature below its melting temperature. For applications requiringhigh accuracy, means may be provided for monitoring the temperature ofthe oscillator housing and correcting the output signal as a function ofthe housing temperature.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates a first embodimentof the applicants invention incorporating a fluidic feedback oscillator;

FIG. 2 illustrates a second embodiment of the applicants inventionincorporating a fluidic sonic oscillator; and

FIG. 3 illustrates a simplified circuit for correcting for errorsintroduced because of a temperature difference between the oscillatorhousing and the fluid flowing therethrough.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, referencenumeral 10 generally identities a first embodiment of the applicantsunique cooled temperature sensor. This temperature sensor embodimentemploys a fluidic feedback oscillator of the type shown in U. S. Pat.No. 3,158,166 issued to R. W. Warren. For the purpose of thisspecification, a

feedback oscillator is defined to be a fluidic oscillator which includesat least one distinct passage for providing feedback signals to switchthe oscillator output signal in an alternating manner.

Oscillator is formed in a housing identified by reference numeral 11which is shown as comprising three plate elements 12, 13 and 14. Plateelement 13 has various openings formed therein and is sandwiched betweenelements 12 and 14. The passages formed through cooperation of elements12, 13 and 14 form the oscillator. These passages include an inletpassage 17, a first feedback passage 19, a second feedback passage 20and a pair of exhaust passages 22. A splitter element 21 is aligned withinlet passage 17 and separates the entrances to exhaust passages 22.

A signal piekoff passage 27 is shown in communication with feedbackpassage 19. An output signal conduit 28 is shown in communication withpiekoff passage 27 to facilitate the connection of any desiredutilization device. It is pointed out that, in operation, oscillatingsignals of the same frequency are produced in feedback passages 19 and20 and exhaust passages 22. Accordingly, although piekoff passage 27 isshown in communication with feedback passage 19, it can equally as wellcommunicate with feedback passage 20 or either of exhaust passages 22.Further, although a specific oscillator configuration is shown, theapplicant does not wish to be limited to this configuration, butcontemplates the use of other feedback oscillator configurations havingsuitable characteristics.

The body of housing 11 is provided with a plurality of cooling passages24 which extend through plate elements 12, 13 and 14. One end of each ofpassages 24 terminates in a manifold 31 formed by element 14 and a coverelement (shown in section). The other end of each of passages 24terminates in a manifold formed by element 12 and a cover element 16.Manifold 31 communicates with a source (not shown) of cooling fluidthrough an inlet 30. The manifold in cover element l6 communicates witha sink (not shown) for the cooling fluid through outlets 40 and 41.Plate element 13 includes provisions at 42 for imbedding a thermocoupletherein. The leads from a thermocouple at 42 are broughtoutside housing11 through a channel 43. The reasons for imbedding a thermocouple inhousing 11 will be discussed hereinafter.

Referring to FIG. 2, reference numeral 50 generally identifies a secondembodiment of the applicants unique cooled temperature sensor.Temperature sensor 50 employs a fluidic sonic oscillator which is formedin a housing 51 comprising plateelement 52, 53 and 54. A sonicoscillator suitable for this useage is disclosed in the applicant'sco-pending application Ser. No. 469,972, filed June 30, 1965 (now U.S.Pat. No. 3,61 3,452 issued Oct. 19, 1971) and assigned to the assigneeof the present invention. For the purpose of this specification, a sonicoscillation is a fluidic oscillator which includes no distinct feedbackpassage, and in which the frequency of oscillation is a function ofcertain characteristic oscillator dimensions.

Plate element 53 has various openings therein, and in combination withelements 52 and 54 forms a plurality of fluid passages and chambers. Thepassages and chambers include an inlet passage 58, a pair of chambers 56and 57 and a pair of exhaust passages 59 and 60. Inlet passage 58 isaligned with a splitter element 65. Chambers 56 and 57 are symmetricallylocated on opposite sides of passage 58 and splitter element 65. Anoutput signal conduit is shown in communication with exhaust passage 60.However, oscillating signals of the same frequency are produced in bothexhaust passages 59 and 60. The output signal conduit can equally aswell be connected to exhaust passage 59. Further, although a specificsonic oscillator configuration is shown in FIG. 2, the applicant doesnot wish to be limited to this configuration, but contemplates the useof other sonic oscillator configurations having suitablecharacteristics. z

The body of housing 51 is provided with a plurality of cooling passages72 which extend through plate elements 52, 53 and 54. One end of each ofpassages 72 terminates in a manifold 71 formed by element 54 and a coverelement 76 (shown partially broken away). The other end of each ofpassages 72 terminates in a manifold formed by element 52 and a coverelement 77. Manifold 71 communicates with a source (not shown) ofcooling fluid through an inlet 70: The manifold in cover element 77communicates with a sink (not shown) for the cooling fluid throughoutlets 74 and 75.

A thermocouple 64 is shown imbedded in housing 51. Reference numerals 66and 67 identify leads from thermocouple 64 which are brought outsidehousing 11. The function of thermocouple 64 will be discussedhereinafter.

The temperature sensors shown in both FIGS. 1 and 2 are speciallyconstructed to prevent bending and warpage of the sensor housings due tolarge thermal gradients therein. Conventional construction techniques,such as bolting and riviting have proved unsatisfactory under extremetemperature conditions. Special construction features which minimizebending or warpage are required because such distortion introduceserrors" into the output signal. Examples of construction features whichmay be employed are forming the sensor housing from one piece ofmaterial as .in a powder forming process, or diffusion bonding theseparate portions of the housing into a single structure.

In operation of the temperature sensor of FIG. 1, hot fluid whosetemperature is to be sensed is introduced through inlet passage 17 andforms a stream'which impinges on splitter element 21. Due to randomfluctuations which are inherent in such a fluid stream and the internalgeometry of the oscillator, unequal fluid entrainment occurs on oppositesides of the stream. This results in deflection of the stream toward theexhaust passage adjacent the area of maximum fluid entrainment. As fluidflows into the exhaust passage, a portion thereof is directed into thenegative feedback passage associated with that exhaust passage. Thisresults in a pressure front which travels through the feedback passageand impinges transversely on the incoming fluid stream. The incomingfluid stream is thereby transferred to the other exhaust passage and asimilar operational sequence occurs. The frequency at which the incomingfluid stream oscillates between the outlet passages is controlled by thelength of the feedback paths and the acoustic velocity in the fluidtherein. The acoustic velocity in the fluid is a function of itstemperature. Accordingly, the frequency of the oscillation set up in theincoming stream is a function ofits temperature. 7

Since the temperature of the incoming fluid may be above the maximumtemperature housing 11 is capable of withstanding, means is provided forcooling the housing. This cooling means comprises manifold13l, themanifold in element 16, cooling passages 24 and means for circulating acooling fluid through the manifolds and cooling passages. It shouldbenotedthat thecooling fluid does notmix with the fluidwhose temperatureis being measured, but only serves to cool the housing.

The primary function of the cooling means is to keep housing 11 at atemperature below its maximum permissible temperature. In manyapplications this may be adequately accomplished by merely circulating arelatively constant flow of cooling fluidatarelatively constanttemperature through passages 24 in housing 11. [n such an application noautomatic control over coolant flow is required.

in other applications, it may be desirable to maintain housing 1 1 at aconstant temperature (for example, just below the maximum permissibletemperature). Alternately, it may be desirable to maintain a constantcoolant flow unless the housing temperature becomes excessive, in whichcase the coolant flow should be increased. In still further applicationsit may be desirable to maintain the housing at a temperature which is afixed amount below the temperature being sensed. In such applications, athermocouple may be provided at 42 to sense the'temperature of housing11. Since the temperature of the housing cannot change rapidly due toits thermal inertia, a thermocouple can adequately respond to suchtemperature changes. The thermocouple output signal may be supplied toany one of a number of suitable conventional flow control systems so asto appropriately control coolant flow through the housing.

In operation of the oscillator of FIG. 2, hot fluid whose temperature isto be sensed is introduced through inlet passage 58 and forms a streamwhich impinges on splitter element 65. Through a phenomena known as theedge-tone effect, which occurs under certain conditions when a fluidstream impinges on a sharp edge splitter, the incoming fluid stream isunstable and tends to oscillate about splitter element 65. Shifting ofthe stream to one side of splitter element 65 results in a small fluidpressure pulse which travels in a direction transverse to the incomingstream. This fluid pulse travels across chamber 56 (or 57) and isreflected back from the opposite chamber wall. The reflected signalproduces curvature in the fluid stream issuing from passage 58 in such amanner as to cause the jet to move to the opposite side of splitterelement 65 and thereby produce a pressure disturbance in the oppositechamber 57 (or 56) which responds in a like'manner, thus causing acomplete oscillation of the fluid stream issuing from passage 58.

The frequency of oscillation of the stream about splitter element 65 isdetermined by the resonant frequency in chambers 56 and 57. The resonantfrequency in these chambers is a function of the chamber dimensions andthe acoustic velocity in the fluid therein. Since the acoustic velocityin the fluid is a function of its temperature, the resonant frequency inchambers 56 and 57, and consequently the frequency of operation of theoscillator, is determined by the temperature of the fluid in thechambers.

Cooling means comprising manifold 71, the manifold in element 77,cooling passages 72 and means for circulating a cooling fluid throughthe manifolds and cooling passages is provided to maintain housing 51 ata temperature below the maximum temperature it is capable ofwithstanding. The operation of this cooling means is identical with theoperation of the cooling means associated with sensor 10 as hereinbeforedescribed. Reference may be made to that description for furtheroperational explanation of the cooling means for sensor 50.

The operations of the oscillators of FIGS. 1 and 2 are only brieflydiscussed above. More detailed discussions of their operations arecontained in above-referenced US. Pat. No. 3,158,166 and patentapplication Ser. No. 469,972 respectively. It should be noted that asthe temperature of the incoming fluid in each of these oscillatorsincreases, the frequency of the output signal produced thereby alsoincreases. Further, it is apparent that if there is a temperaturedifference between the fluid whose temperature is being sensed and thesensor housing, there will be heat transfer between the sensor housingand the fluid therein. Consequently, the frequency of oscillation in thesensor will depend on the temperature of the sensor housing. Theapplicant has, however, discovered that, for suitable oscillators underthe proper operating conditions, this effect is unexpectedly small.

The capability of the applicants temperature sensor to accurately sensetemperature while in a cooled condition stems primarily from the basicphenomena on which it operates. This phenomena is that the acousticvelocity in a fluid is directly dependent on the temperature of thefluid. Thus, the primary demand made on the housing of a fluidictemperature sensor employing this phenomena is structural integrity. Itis apparent that if the housing material must be cooled below thetemperature being sensed to maintain its structural integrity, therewill be a temperature transition region in the fluid at any point it isin contact with the sensor housing. If this transition region orboundary layer is substantial, it will have a substantial effect on theaverage acoustic velocity in the fluid within the sensor.

The boundary layer thickness is a function of various parametersincluding the temperature difference between the sensor housing and theincoming fluid, and the velocity and other flow characteristics of fluidwithin the sensor. For normal flow velocities through the sensors ofFIGS. 1 and 2, the thickness of the boundary layer is quite small(typically .05 inches) If the pressure differential across the sensorand the sensor geometry are such as to maintain turbulent flow therein,the boundary layer thickness may be even smaller. Further, underturbulent flow conditions, a relatively uniform boundary layer thicknessis maintained throughout the sensor. Since this boundary layer thicknessis small in comparison to a characteristic fluid path length (in theorder of l to 2 inches) within the sensor, the fact that the boundarylayer has a different temperature than the incoming fluid has arelatively insignificant effect on the frequency of the sensor outputsignal. Thus, the accuracy of the sensor under cooled conditions is notseriously impaired.

It should further be pointed out that the applicants temperature sensorpossesses fast time response. This characteristic is also due primarilyto the fact that the output signal frequency is dependent upon thetemperature of the fluid within the sensor. Under normal operatingconditions, the purging time of the sensor (that is, the time requiredfor complete replacement of the fluid in the sensor) is in the order ofa few milliseconds. Since all that is required to change the outputsignal frequency is that the temperature of the fluid within the sensorbe changed, fast time response can be achieved with the applicantscooled temperature sensor.

in addition to the desirable characteristics of good accuracy and fasttime response, the applicants sensor is of basically simpleconstruction. Further, its ruggedness and reliability are only limitedby the characteristics of the material from which it is constructed.

Although the existence of a temperature differential between the sensorhousing and the fluid whose temperature is being measured does notintroduce a substantial error in the sensor output signal, it isapparent that some error will be introduced. For applications requiringa highly accurate sensor, compensation for this error can be providedwith relative simplicity. A block diagram of one system suitable forcompensating for this error is shown in FIG. 3. As in FIG. 2, referencenumeral 50 identifies a fluidic oscillator of the sonic type. Oscillator50 is shown for purposes of illustration only. Feedback oscillator 10 ofFIG. 1 may readily be substituted for oscillator 50.

ln FIG. 3, fluid whose temperature is to be measured is supplied to theinlet passage in oscillator 50 by means of a conduit 80. Within theoscillator, this fluid is set into oscillation in the mannerhereinbefore described. The oscillating output signal produced'byoscillator 50 is transmitted by means of conduit 55 to apneumaticelectric pressure transducer 81. Transducer 81 may be of anysuitable type. One such suitable device is a capacitance transducerwherein a diaphragm forms one plate of a capacitor. The pressure pulsetrain output signal from oscillator 50 causes the diaphragm to deflectin accordance with the pressure pulses, thus varying the capacitancepresented by the transducer.

The plates of the capacitor within transducer 81 may be connected to afrequency to level converter 82 by means of a pair of conductors 83.Converters suitable for this application are well known. One suchsuitable converter is General Radio model 1145A. The output signal ofconverter 82 is an analog electrical signal whose magnitude bears alinear relationship to the frequency of the output signal fromoscillator 50. It should, however, be noted that the output signal fromconverter 82 is not a linear function of the temperature of the fluidentering oscillator 50. The reason for this is that the acousticvelocity in a fluid varies as the square root of its temperature.Accordingly, the frequency of the output signal produced by oscillator50 and the magnitude of the output signal produced by. converter 82 varyas the square root of the temperature of the fluid in oscillator 50.

it is frequently advantageous to have a signal which is a linearfunction of the fluid temperature being measured. To accomplish thisresult, the output signal from converter 82 is transmitted to alinearizer 84 by means of a pair of conductors 85. Linearizer 84functions to transform the output signal produced by converter 82 intoan analog electrical signal whose magnitude varies linearly with thefluid temperature being measured. A number of suitable linearizingcircuits are available. In its simplest form, linearizer 84 comprises anetwork of resistors sized to achieve the desired linearizing function.

If the housing temperature of oscillator 50 and the temperature of theentering fluid are equal, the magnitude of the output signal produced bylinearizer 84 accurately indicates the entering fluid temperature.However, in the event that the two temperatures are not equal, it may bedesirable to measure the housing temperature and provide a correctionfor the temperature differential. Thermocouple 64 is provided to sensethe housing temperature. A voltage indicative of the housing temperatureis transmitted to a thermocouple amplifier 86 by means of leads 66 and67. The voltage produced between leads 66 and 67, however, is not alinear function of the housing temperature. Consequently, the outputsignal from amplifier 86 is transmitted to a second linearizer 87 bymeans of a pair of conductors 88.

The output signal of linearizer 87 is transmitted to a first input of anon-linear function generator 90 by means of a pair of conductors 93.Such function generators are well known. One simple configuration maybasically comprise a loaded vacuum tube. Function generator 90 alsoincludes a second input at which it receives the output signal fromlinearizer 84 through pairs of conductors 91 and 92. Function generator94 takes the difference between signals indicative of the housingtemperature of oscillator 50 and the temperature of the fluid thereinand computes a correction factor. A signal indicative of this correctionfactor is transmitted to a first input of a summing circuit 94 by meansof a pair of conductors 95. Summing circuit 94 includes a second inputat which it receives the output signal from circuit 84 throughconductors 91. Circuit 94 serves to sum the signal indicative of thecorrection factor with the signal indicative of the temperature of thefluid within oscillator 50 and produce a signal which bears an accuratelinear relationship to the temperature of the fluid entering oscillator50.

Specifically, if the temperature being measured and the housingtemperature of oscillator 50 are equal, the magnitudes of the outputsignals from linearizers 84 and 87 will be equal and no signal will besupplied from function generator 90 to summing circuit 94. Thus, themagnitude of the output signal from circuit 94 will be equal to themagnitude of the signal produced by linearizer 84. No correction of thesignal produced by linearizer 84 is necessary and none is provided.However, if the housing temperature of oscillator 50 is higher or lowerthan the temperature being sensed, the signal produced by functiongenerator 90 will be negative or positive signal whose magnitude isindicative of the difference between the two temperatures. Functiongenerator 90 will accordingly supply an appropriate correction signal tocircuit 94. This signal is used within circuit 94 to modify the signalsupplied thereto from circuit 84 so as to provide an appropriatecorrection and produce an output signal whose magnitude bears anaccurate linear relationship to the actual temperature of the fluidentering oscillator 50.

Reference numeral 100 identifies a utilization device requiring ananalog electrical signal whose magnitude is accurately indicative of ameasured temperature. The output signal from summing circuit 94 istransmitted to device 100 through a pair of conductors 99. Device 100may comprise any one of a number of well known indicators or controlsystems.

What is claimed is: 1. Fluidic oscillator sensing apparatus capable ofwithstanding very high temperatures comprising:

housing means enclosing passages which form a fluidic oscillator throughwhich a fluid having a variable parameter may flow, the fluidicoscillator being operable to produce an output signal whose basicfrequency is determined by the length of a characteristic acousticsignal path therein and whose frequency varies in accordance withvariations in the variable parameter, said housing means having an upperstructural temperature limit; cooling means for maintaining said housingmeans at a temperature below said temperature limit even though thetemperature of fluid flowing through said fluidic oscillator issubstantially higher than said temperature limit; sensor means forproducing a compensation signal indicative of the temperature of saidhousing means; and correction means connected to receive said outputsignal and said compensation signal, said correction means for producinga signal indicative of said output signal modified by a factor dependenton said compensation signal so as to substantially reduce any error duetothe existence of a temperature difference between said housing meansand the fluid flowing through said fluidic oscillator.

2. The apparatus of claim 1 wherein said fluidic oscil- 5 lator is asonic oscillator.

3. The apparatus of claim 2 wherein said cooling means comprisespassages in said housing means separated from said fluidic oscillatorand means for circulating a cooling fluid through said passages.

4. The apparatus of claim 3 wherein: said sensor means comprises athermocouple for producing a first electrical signal; and

said correction means includes transducer means for lator is a feedbackoscillator.

6. The apparatus of claim 5 wherein said cooling means comprisespassages in said housing means separate from said fluidic oscillator andmeans for circulating a cooling fluid through said passages.

7. The apparatus of claim 6 wherein:

said sensor means comprises a thermocouple for producing a firstelectrical signal; and

said correction means includes transducer means for converting saidoutput signal into a second electrical signal and means for modifyingsaid second electrical signal in response to said irst electricalsignal, the modified electrical signal being accurately representativeof the variable parameter.

1. Fluidic oscillator sensing apparatus capable of withsTanding veryhigh temperatures comprising: housing means enclosing passages whichform a fluidic oscillator through which a fluid having a variableparameter may flow, the fluidic oscillator being operable to produce anoutput signal whose basic frequency is determined by the length of acharacteristic acoustic signal path therein and whose frequency variesin accordance with variations in the variable parameter, said housingmeans having an upper structural temperature limit; cooling means formaintaining said housing means at a temperature below said temperaturelimit even though the temperature of fluid flowing through said fluidicoscillator is substantially higher than said temperature limit; sensormeans for producing a compensation signal indicative of the temperatureof said housing means; and correction means connected to receive saidoutput signal and said compensation signal, said correction means forproducing a signal indicative of said output signal modified by a factordependent on said compensation signal so as to substantially reduce anyerror due to the existence of a temperature difference between saidhousing means and the fluid flowing through said fluidic oscillator. 2.The apparatus of claim 1 wherein said fluidic oscillator is a sonicoscillator.
 3. The apparatus of claim 2 wherein said cooling meanscomprises passages in said housing means separated from said fluidicoscillator and means for circulating a cooling fluid through saidpassages.
 4. The apparatus of claim 3 wherein: said sensor meanscomprises a thermocouple for producing a first electrical signal; andsaid correction means includes transducer means for converting saidoutput signal into a second electrical signal and means for modifyingsaid second electrical signal in response to said first electricalsignal, the modified electrical signal being a signal accuratelyrepresentative of the variable parameter.
 5. The apparatus of claim 1wherein said fluidic oscillator is a feedback oscillator.
 6. Theapparatus of claim 5 wherein said cooling means comprises passages insaid housing means separate from said fluidic oscillator and means forcirculating a cooling fluid through said passages.
 7. The apparatus ofclaim 6 wherein: said sensor means comprises a thermocouple forproducing a first electrical signal; and said correction means includestransducer means for converting said output signal into a secondelectrical signal and means for modifying said second electrical signalin response to said first electrical signal, the modified electricalsignal being accurately representative of the variable parameter.